In many oil wells, the annular space or annulus between the well casing or liner and the subterranean formation is filled with cement. The cement protects the oil and/or gas-producing zones surrounding the casing from contamination by subterranean brine and helps prevent corrosion of the casing or liner by other corrosive fluids and electrolysis. Cementing also helps protect any fresh water-containing strata from contamination by the brine or by petroleum fluids that could otherwise travel in the wellbore annulus. Additional benefits of cementing include bonding of the well casing to the formation or to other well casings to support any vertical and radial loads applied to the casings. For at least these reasons, it is useful to be able to determine whether there is cement in the annular space and the quality of the cement, or whether there is some other material, such as barite, air, or the like filling the annular space.
Cement quality evaluations are traditionally conducted downhole by running cement bond logging (CBL) tools in a cased wellbore. Other evaluation methods include running an open hole sonic log and cased hole neutron log to detect fluid channels in the cement. The cased hole neutron log typically requires information regarding the porosity of the formation, as porosity may affect how easily neutrons travel through the formation to the detectors. The open and cased hole logs, hole dimensions, and related information are then provided as input to a mathematical model generated on a computer to determine a volume of fluid filled channels in the cement sheath and estimate the cement integrity based on the fluid filled channel volume.
Other techniques exist for evaluating the materials within the annular space between a well casing and a subterranean formation or between two well casings. Nevertheless, a need exists for an improved way for evaluating such annulus materials.